The present invention relates to an electrothermal transfer sheet, and more particularly to a thermal transfer sheet for use in an electrothermal transfer printing system.
The electrothermal transfer method is a method in which an electric current is applied to a transfer sheet from an electrode head to generate heat, and transfer recording of an image is effected by utilizing this heat. In this method, an electrothermal transfer sheet composed of a substrate sheet, a resistor layer capable of generating heat when an electric current is applied thereto from an electrode head, formed on one surface of the substrate sheet, and a dye layer which is a sublimable dye layer or a wax ink layer dyed with a pigment, formed on the other surface of the substrate sheet has been conventionally used as the transfer sheet.
In the electrothermal transfer method, as described above, thermal energy is generated by applying an electric current to the resistor layer of the electrothermal transfer sheet from an electrode head, and the thus generated heat is utilized for transfer recording of an image. Concentration of heat is therefore readily caused in the electrothermal transfer sheet, and the resistor layer partially has an extremely high temperature. As a result, the resistor layer is fused or softened, and the electrothermal transfer sheet and the electrode head are adhered to each other, or scrapings of the resistor layer deposit on the electrode head, causing a short circuit, whereby the electrothermal transfer sheet is broken. Thus, the conventional electrothermal transfer sheet has the problems concerning resistance to heat.
To improve the heat resistance of the resistor layer which is provided on the substrate sheet, one of the following conventional methods has been adopted:
(a) a method in which a resistor layer is prepared using a resin having high resistance to heat;
(b) a method in which a resistor layer is hardened by application of heat, using a crosslinking agent such as polyisocyanate, thereby imparting heat resistance to the resistor layer; and
(c) a method in which a reactive monomer is incorporated into a resistor layer and crosslinked by application of an ionizing radiation, or a resistor layer is prepared using an ionizing-radiation-curable resin, thereby imparting heat resistance to the resistor layer.
The above methods (b) and (c) are disclosed in Japanese Laid-Open Patent Publication No. 283495/1990. With respect to the method (a), resins having high resistance to heat are generally expensive. In addition, they cannot be readily dissolved in commercially available widely-used solvents, so that films cannot be easily formed when such resins are employed. When aromatic polyisocyanate is used in the method (b), the resistor layer is hardened rapidly, so that it tends to shrink. Such a shrinkage is unfavorable because the thermal transfer sheet acquires wrinkles. In the case where aliphatic polyisocyanate is used, the resistor layer is hardened slowly (3 to 7 days at 40.degree. C.). This affects the process which comes after this hardening process, and also increases the production cost. Further, in this case, it is required to make the crosslinking agent a two-part system, so that the resistance value of the resistor layer becomes large. The resistor layer formed in the method (c), crosslinked by an ionizing radiation, exhibits reduced adhesion to the substrate sheet. Moreover, an adhesive resin which can improve the adhesion between such a resistor layer and the substrate sheet is very few and limited.